[parappa]

I'd raise preflop, and since everyone's in I think that I'd call the flop and raise a turn blank.

Since you describe MP3 as passive, I tend to credit this bet more than most people do. Against someone that I have enough hands with, I'll be able to conclude I'm behind. I still think the turn raise is good in this spot, though.

I think the river bet's okay, with the same disclaimer as the turn. If I strongly suspected that MP3 wouldn't bet without a hand that beats me, I'd just show it down.